Burner design for particle generation

ABSTRACT

A method of producing bi-modal particles includes the steps of igniting a first precursor gas using a primary burner thereby producing a first plurality of particles of a first size, fluidly transporting the first plurality of particles down a particle tube, igniting a second precursor gas using a secondary burner thereby producing a second plurality of particles of a second size, flowing the second plurality of particles into the first plurality of particles, and capturing the first and second plurality of particles.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 62/310,309 filed on Mar. 18, 2016the content of which is relied upon and incorporated herein by referencein its entirety.

BACKGROUND

The present invention generally relates to methods and apparatuses forforming particles, and more particularly, relates to the simultaneousproduction of particles having a bimodal size distribution.

Conventional manufacturing processes for producing optical fiberspreforms include growing the preform by utilizing an outside vapordeposition technique where silica soot particles are sprayed onto thepreform as it is rotated. Advances in the production of optical fiberpreforms now make it capable to produce a plurality of silica sootparticles for later pressing into a green body. The green body may laterbe consolidated into an optical fiber preform.

SUMMARY

According to one embodiment, a method of producing bi-modal particlesincludes the steps of igniting a first precursor gas using a primaryburner thereby producing a first plurality of particles of a first size,fluidly transporting the first plurality of particles down a particletube, igniting a second precursor gas using a secondary burner therebyproducing a second plurality of particles of a second size, flowing thesecond plurality of particles into the first plurality of particles, andcapturing the first and second plurality of particles.

According to another embodiment a burner system includes a primaryburner positioned within a particle tube. The primary burner isconfigured to produce a first plurality of particles. A secondary burneris positioned within a secondary tube. The secondary burner isconfigured to produce a second plurality of particles. The particle tubeand the secondary tube are fluidly connected such that the firstplurality of particles and the second plurality of particles are mixed.A bag house is configured to collect the first and second plurality ofparticles.

According to yet another embodiment, a method of forming an opticalfiber preform for making an optical fiber includes steps of igniting agas using a primary burner thereby producing a first plurality ofparticles of a first size,

igniting a second gas using a secondary burner thereby producing asecond plurality of particles of a second size, flowing the secondplurality of particles into the first plurality of particles, capturingthe first and second plurality of particles, and pressing the first andsecond plurality of particles into an optical fiber preform.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiments, and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a particle production system,according to one embodiment;

FIG. 2 depicts a top perspective view of a primary burner of theparticle production system, according to one embodiment;

FIG. 3 depicts a top perspective view of a secondary burner of theparticle production system, according to one embodiment;

FIG. 4 is a flowchart illustrating a use of a soot generated by theparticle production system, according to one embodiment;

FIG. 5 is a graph depicting the attributes of particles produced basedon time spent at an elevated temperature;

FIG. 6 depicts a fluid flow simulation of a burner configurationutilizing both a large particle burner and a small particle burner;

FIG. 7 depicts an the tensile strength of green bodies formed fromparticles made according to the present disclosure; and

FIG. 8 depicts the strain tolerance of green bodies formed from particlemade according to the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiments,examples of which are illustrated in the accompanying drawings. Wheneverpossible, the same reference numerals will be used throughout thedrawings to refer to the same or like parts.

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof, shall relate to the disclosure as oriented in FIG. 1, unlessstated otherwise. However, it is to be understood that the disclosuremay assume various alternative orientations, except where expresslyspecified to the contrary. It is also to be understood that the specificdevices and processes illustrated in the attached drawings, anddescribed in the following specification, are simply exemplaryembodiments of the inventive concepts defined in the appended claims.Hence, specific dimensions and other physical characteristics relatingto the embodiments disclosed herein are not to be considered as limitingunless the claims expressly state otherwise. Additionally, embodimentsdepicted in the figures may not be to scale or may incorporate featuresof more than one embodiment.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

Referring now to FIG. 1, depicted is schematic representation of aparticle production system 10 configured to produce a plurality ofparticles for collection. The particle production system 10 includes aplurality of burners, each burner configured to generate a plurality ofparticles through combustion of a precursor gas (i.e. a gas precursorwhich is used to form the constituents of the particles). In thedepicted embodiment, the particle production system 10 includes aprimary burner 14 and a secondary burner 18. It will be understood thatthe particle production system 10 may be scaled up to include more thanone primary burner 14 or more than one secondary burner 18.Additionally, the particle production system 10 may include a greaternumber of secondary burners 18 than primary burners 14, or vice versa.As explained in greater detail below, the primary burner 14 isconfigured to produce a first plurality of particles 20A and thesecondary burner 18 is configured to produce a second plurality ofparticles 20B. In various embodiments, the first and second pluralitiesof particles 20A, 20B may have different particle sizes, distributionsof sizes, compositions, morphologies and/or quantities produced.

In the depicted embodiment of the particle production system 10, theprimary burner 14 is coupled to a particle tube 22 configured totransport the first plurality of particles 20A generated by the primaryburner 14 to a baghouse 26. The secondary burner 18 is positioned on asecondary tube 30 which is fluidly connected with the particle tube 22such that the second plurality of particles 20B generated by thesecondary burner 18 may flow into the stream of the first plurality ofparticles 20A generated by the primary burner 18. Although the secondaryburner 18 is depicted as downstream of the primary burner 14, it will beunderstood that the primary burner 14 may be positioned downstream ofthe secondary burner 18. As explained above, the first and secondpluralities of particles may have different particle sizes (e.g.,diameter or longest length), distributions of sizes, compositions,morphologies and/or quantities produced. For example, the differences inproperties of the first and second particles may lead to the sootparticles collected in the baghouse 26 to have a bi-modal distributionin size and/or morphology. In another example, the weight percentage ofthe second plurality of particles 20B in the mixture of first and secondplurality of particles 20A, 20B may be greater than about 1%, 2%, 3%,4%, 5%, 6%, 7%, 8%, 9%, 10%, 25%, 50%, 75%, 90% or greater than about99%. The baghouse 26 is configured to allow an exhaust stream from theprimary and secondary burners 14, 18 to pass though it while collectingthe first and second plurality of particles 20A, 20B. The baghouse 26may include a plurality of modules incorporating filter bags. Theexhaust carrying the first and second pluralities of particles 20A, 20Bis passed through the filter bags which collects the particles 20A, 20B,but allows the exhaust gas to pass through and expelled.

Extending from the secondary burner 18 is a plurality of gas lines 40which fluidly connect a gas supply 44 with the secondary burner 18. Theplurality of gas lines 40 include a shield gas line 40A, a precursor gasline 40B and a premix line 40C. The gas supply 44 may include aplurality of gas sources configured to provide a shield gas, a precursorgas, a combustible gas mixture, and all the constituents thereof asexplained in greater detail below. The plurality of gas lines 40 may becoupled to one or more gas flow restrictors 48. The flow restrictors 48may include one or more thermal mass flow controllers configured tocontrol the flow rate of one or more of the constituents of the a shieldgas, a precursor gas and/or a combustible gas mixture. In someembodiments, the flow restrictor 48 may include upwards of seven or morethermal mass flow controllers (e.g., one for each constituent of theshield gas, a precursor gas and/or a combustible gas mixture). The gasflow restrictors 48 may be controlled by a controller 52 to limit theamount of pressure or flow rate of gas that reaches the secondary burner18 from the gas supply 44. The controller 52 may be electrically coupledto one or more temperature and pressure sensors positioned along the gaslines 40 and within the secondary burner 18 and configured to alter theindividual flow rates of the gas lines 40 to optimize the performance ofthe burner 18. It will be understood that the primary burner 14 mayinclude a similar gas supply, flow restrictor and gas line as thatdescribed in connection with the secondary burner 18.

Referring now to FIG. 2, the primary burner 14 is configured to producethe first plurality of particles 20A through the combustion of theprecursor gas. In the depicted embodiment, the primary burner 14includes a honeycomb structure 60 and a nozzle 64. The honeycombstructure 60 may provide gas turbulence to a carrier gas passed throughthe honeycomb structure 60. The nozzle 64 includes a plurality ofsubstantially concentric tubes 68 through which a plurality of gases mayflow. For example, the centermost tube may pass a precursor gas mixturewhich may include a particle precursor compound (e.g.,octamethylcyclotetrasiloxane, silicon tetrachloride, tetraethylorthosilicate, etc.), an oxidizer (e.g., diatomic oxygen, organicoxidizers, etc.) and a combustible gas (e.g., hydrogen, natural gas,methane, etc.). Each of the concentric tubes 68 may pass a separate gas(e.g., diatomic oxygen, methane, diatomic nitrogen, carrier/shield gas)or a combination of gasses. Combustion and/or heating of the precursorgas proximate the nozzle 64 creates the first plurality of particles 20Ain a soot or gaseous like state suspended in the combustion exhaust andthe carrier gas. The expansion of the gasses during combustion and flowof the carrier gas then carry the first plurality of particles 20A in asoot stream down the particle tube 22 toward the secondary burner 18 andthe baghouse 26. The first plurality of particles 20A may exhibit a D50diameter of greater than about 25 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm,85 nm, 90 nm, 100 nm or greater than about 110 nm. D-values are used todescribe particle size distributions. A D-value can be thought of as a“mass division diameter.” It is the diameter which, when all particlesin a sample are arranged in order of ascending mass, divides thesample's mass into specified percentages. The percentage mass below thediameter of interest is the number expressed after the “D.” Thus the D50diameter of a collection of particle samples is the diameter at which50% of a sample's mass is composed of smaller particles. The D50 is alsoknown as “mass median diameter” as it divides the sample equally bymass. Depending on the precursor gas used, the first plurality ofparticles 20A may be composed of silicon, carbon, organics and/or othercompounds. In an exemplary embodiment, the first plurality of particles20A may be formed of SiO₂. In some embodiments, the precursor gas mayinclude a doping agent (e.g., rare earth ions or halogens) such that thefirst plurality of particles 20A may be doped.

Referring now to FIG. 3, once produced, the first plurality of particles20A (FIG. 1) travel through the particle tube 22 (FIG. 1) in a soot orgaseous state toward the secondary burner 18. The secondary burner 18may be a different construction than that of the primary burner 14 (FIG.2). The secondary burner 18 includes a body 80 defining an annulus 84through which a precursor tube 88 extends. In the depicted embodiment,the precursor tube 88 may extend coaxially with the annulus 84, but inother embodiments may be off centered. Defined by the body 80, anddisposed around the annulus 84, is a plurality of pilot holes 92. In thedepicted embodiment, the body 80 defines four groupings of two pilotholes 92, but in various embodiments may include more than eight or lessthan eight pilot holes 92 in any configuration (e.g., grouped or equallyspaced). In one embodiment, the pilot holes 92 may have a diameter ofabout 0.04 in (1.02 mm), the precursor tube 88 may have a diameter ofabout 0.25 in (6.35 mm). The annulus 84 may have a diameter of about0.348 in (8.84 mm). The pilot holes 92 may have a center-to-centerspacing from the precursor tube 88 of about 0.22 in (5.59 mm). In thedepicted embodiment, each grouping of pilot holes 92 may have acenter-to-center distance of about 0.08 in (2.03 mm).

The pilot holes 92 are configured to pass a combustible gas mixturethere through which may be ignited. The combustible gas mixture mayinclude diatonic oxygen, diatomic hydrogen and a flammable organic gas(e.g., natural gas or methane). The combustible gas mixture may beregulated and mixed in the gas flow restrictor 48. The combustible gasmixture may have a diatomic oxygen flow rate of between about 0.1standard liters per minute (SLPM) and about 1.0 SLPM, or between about0.4 SLPM and about 0.6 SLPM. The combustible gas mixture may have adiatomic hydrogen flow rate of between about 0.1 SLPM to about 2.0 SLPM,or between about 1.00 SLPM to about 1.6 SLPM. The flammable organic gasmay have a flow rate of between about 0.1 SLPM and about 0.2 SLPM. Inone example, the premix may have 0.175 SLPM of methane, 1.05 SLPM ofdiatomic hydrogen and 0.5 SLPM of diatomic oxygen. In another example,the premix may have 0.175 SLPM of methane, 1.072 SLPM of diatomichydrogen and 0.455 SLPM of diatomic oxygen. In yet another example, thepremix may have 0.175 SLPM of methane, 1.072 SLPM of diatomic hydrogenand 0.455 SLPM of diatomic oxygen. It will be understood that the aboveranges on flow rates are disclosed for a test scale apparatus and thatthe scale of the premix mixture may be increased with approximately thesame ratio disclosed, or with varying ratios, without departing from thespirit of the disclosure.

The precursor tube 88 of the secondary burner 18 is configured to pass aprecursor gas therethrough. The annulus 84 positioned around theprecursor tube 88 is configured to pass a shield or carrier gas aroundthe precursor gas. In various embodiments, the shield or carrier gas maybe inert to the precursor gas (e.g., diatomic nitrogen, noble gas,carbon dioxide). Typically, oxygen (e.g., from the surrounding air orenvironment) should not contact the precursor gas during the combustionprocess. If oxygen contacts the precursor gas, the resulting soot streamof the second plurality of particles 20B may touch down on the body 80of the secondary burner 18 and deposit particles around precursor tube88. Passing an inert gas, such as nitrogen, through the annulus 84 toform a shield around the precursor gas moves the reaction between theprecursor gas and oxygen to a point sufficiently far from the secondaryburner 18 to prevent soot deposition on the precursor tube 88. Theprecursor gas is configured to be ignited and combusted such that thesecond plurality of particles 20B is formed therefrom in a soot orgaseous form. The combustion of the precursor gas may form aself-sustaining flame such that the pilot holes 92 are, in essence, asafety feature and are optional. The precursor gas, which is passedthrough precursor tube 88, may include diatomic hydrogen, diatomicoxygen and an organic siloxane. The organic siloxane may includeoctamethylcyclotetrasiloxane ([(CH₃)₂SiO]₄) (OMCTS) or other siloxanesbonded with organic molecules. In another embodiment, the precursor gasmay include Tetraethyl orthosilicate. In a specific example, theprecursor gas of the secondary burner 18 may include about 41 SLPM ofdiatomic oxygen, between about 4.0 SLPM and about 13 SLPM of diatomichydrogen and between about 0.5 g/min and about 3.0 g/min of OMCTS. Itwill be understood that the above ranges on flow rates are disclosed fora test scale apparatus and that the scale of the precursor gas may beincreased with approximately the same ratio disclosed, or with varyingratios, without departing from the spirit of the disclosure.

The second plurality of particles 20B resulting from the combustion mayexhibit a primary particle size, or a D50 diameter, of less than about25 nm, 20 nm, 15 nm, 10 nm, 6 nm, 5 nm. It will be understood that theD50 diameter of the second plurality of particles 20B is to bedistinguished from the size of aggregates formed from a plurality of thesecond particles 20B. The second plurality of particles 20B may exhibita surface area of at least 100 m²/g, 200 m²/g, 225 m²/g, 300 m²/g or 350m²/g. Depending on the precursor gas used, the second plurality ofparticles 20B may be composed of silicon, carbon, organics and/or othercompounds. In an exemplary embodiment, the second plurality of particles20B may be formed of SiO₂. In some embodiments, the precursor gas mayinclude a doping agent (e.g., boron, germanium, erbium, titanium,aluminum, lithium, potassium, bromine, cesium, phosphorus, sodium,neodymium, bismuth, antimony, ytterbium) such that the second pluralityof particles 20B may be doped. Combustion of the precursor gas createsan elevated temperature zone (e.g., a gradient of temperatures with thehottest point being proximate the combustion and the coldest point beingdistant from the flame) extending away from the secondary burner 18.

During formation of the particles (e.g., the second plurality ofparticles 20B), the D50 particle diameter is a function of thetemperature of the combustion of the precursor gas and the time theparticles spend at an elevated temperature (e.g., above about 500° C.).Formation of sub 50 nm particles typically is accomplished through acombination of low burner temperature and very short residence timewithin an elevated temperature zone. During formation of the secondplurality of particles 20B, particle constituent molecules (e.g., SiO₂)nucleate into tiny particles and grow with the absorption of andcollision with other constituent molecules. When the second plurality ofparticles 20B grow to a sufficient size, they collide to form clusters.The clusters can sinter at elevated temperatures to form large sphericalparticles. The relationship between the time a particle may spend in anelevated temperature based on a desired size is given by equation (1):

4×10⁻¹² ∫e ^(0.0184T(t)) dt=Z  (1)

where T is the temperature in degrees Celsius and time is measured inseconds. The integration is done over the time at which the temperatureis above 500° C. When a particular time and temperature is chosen suchthat the value of Z=1, the D50 diameter of the particles is about 6 nmand the surface area is about 375 m²/g. When the value of Z is 2, theD50 diameter of the particles is about 10 nm and the surface area isabout 225 m²/g. When the value of Z is 5, the D50 diameter of theparticles is about 30 nm and the surface area is about 90 m²/g. Inspecific examples, to obtain a particle of about 30 nm, a particle maybe at a temperature of about 1650° C. for about 0.04 seconds, at 1550°C. for about 0.1 seconds, 1400° C. for about 1.5 seconds or above about1300° C. for less than about 10 seconds. In specific examples, thesecond plurality of particles 20B may spend no more than about 10seconds, 9 seconds, 8 seconds, 7 seconds, 6 seconds, 5 seconds, 4seconds, 3 seconds, 2 seconds, less than 1 second, less than 0.1 secondswithin the elevated temperature zone.

Control of the temperature of combustion (i.e. and therefore thetemperature of the elevated temperature zone) and the residency time ofthe particles (e.g., the second plurality of particles 20B) within theelevated temperature zone may be controlled by controlling the flow rateof the precursor gas through the precursor tube 88. At low flow rates,heat from the combustion of the of the precursor gas is allowed toincrease (e.g., above 1300° C., above 1400° C., above 1500° C., above1600° C., above 1700° C. or above 1800° C.) and the time it takes thesecond plurality of particles 20B to move through the elevatedtemperature zone may also increase (e.g., greater than about 0.5seconds, greater than about 1.0 seconds, greater than about 10 seconds).As the flow rate of the precursor gas is increased, the heat of thecombustion may be dissipated and/or the combustion of the precursor gasmay be incomplete leading to a lower combustion temperature (e.g., lessthan about 1600° C., less than about 1500° C., less than about 1400° C.or less than about 1300° C.). Additionally, as the precursor gas flowrate increases, the residency time of the second plurality of particles20B within the elevated temperature zone decreases as the speed of theparticles 20B increases. As such, the temperature of the combustion andthe residency time of the particles 20B, and therefore the size of theparticles 20B, may be controlled via the controller 52 by controllingthe flow rate of the precursor gas using the gas flow restrictor 48.Accordingly, by increasing the precursor gas flow rate, the controller52 may decrease the particle size, and by decreasing the precursor gasflow rate, the controller 52 may increase the particle size.

After generation of the second plurality of particles 20B, the particles20B flow away from the secondary burner 18, down the secondary tube 30and flow into the stream of the first plurality of particles 20A withinthe particle tube 22. The secondary tube 30 may be positioned downstreamof the primary burner 14 sufficiently far enough that the secondplurality of particles 20B is mixed into the first plurality ofparticles 20A out of the elevated temperature zone associated with theprimary burner 14 such that sintering between the first and secondpluralities of particles 20A, 20B does not occur. The flow of the secondplurality of particles 20B into the first plurality of particles 20A,while both are carried by the exhaust and shield/carrier gasses, allowsfor an effective mixing the particles 20A, 20B at a molecular level. Itwill be understood that as described herein, mixing at the molecularlevel means mixing the particles sufficiently that there are not clumpsof the second plurality of particles 20B in the gaseous cloud of thefirst plurality of particles 20A. The mixed first and second pluralityof particles 20A, 20B then move down the particle tube 22 and arecollected in the baghouse 26.

The production of the first plurality and second plurality of particles20A, 20B at different diameters allows of the collection in the baghouse26 of a bimodal silica soot. The term bimodal means that there is atleast one minimum between two maxima of the particle size distribution.For example, as explained above, the primary burner 14 may produce thefirst plurality of particles 20A with a D50 diameter of greater thanabout 50 nm, and the secondary burner 18 may produce the secondplurality of particles 20B at a D50 diameter of less than about 30 nm(e.g., about 7 nm), thus providing two maxima separated by a minimum.

Referring now to FIG. 4, an exemplary method 100 is depicted a use ofthe bimodal soot (e.g., the first and second pluralities of particles20A, 20B) produced by the particle production system 10. The method 100may include a step 104 of pressing the soot into an optical fiberpreform, a step 108 of consolidating the optical fiber preform and astep 112 of drawing an optical fiber from the optical fiber preform. Inoptical fiber preform embodiments, the bimodal soot may be silica soot.

Step 104 of pressing the soot may utilize a radial press process toapply one or more layers of the soot, concentrically, to a substrate toform the optical fiber preform. The substrate may include a core region,which may be consolidated or unconsolidated core glass, without orwithout a layer of outside vapor deposited soot. In some embodiments,the substrate may include an optical fiber preform of un-sintered soot.In some embodiments, the layers of soot may include a binder. Theresultant body is a monolithic silica soot blank containing one layer ormultiple concentric layers of pressed soot with a core. In multilayerembodiments, by selecting the porosity, surface area, and/or sootdensity of the soot used to form each of the pressed concentric layers,a multilayered, monolithic soot blank may be formed that includes outercladding layers and optionally an inner cladding which have the same ordifferent physical properties such as porosity, surface area, and/orsoot density. Dopants may be added to the soot and may include, but arenot limited to, chlorine, via use of Cl₂ or SiCl₄, fluorine, via use ofSiF₄, SF₆, or CF₄, and phosphorus via use of POCl₃ or PCl₃ orcombinations thereof. Optionally, the layers of pressed silica soot mayhave different chemical compositions, thus resulting in differentrefractive indexes across the various regions of pressed silica oncethose regions are consolidated. Soot compositions could includefluorine, boron, germanium, erbium, titanium, aluminum, lithium,potassium, bromine, cesium, chlorine, phosphorus, sodium, neodymium,bismuth, antimony, ytterbium, and combinations of these dopants, amongstother dopants in a silica matrix.

Pressing of the soot may include a series of radial pressing steps, eachadding in sequence each radial segment of soot to form multiplesequential layers of soot differing in physical or compositionalproperties. Alternatively, more than one distinct layer can be providedin a single radial pressing step by fitting removable dividers into thecavity of a mold outside of the substrate, and extending the full axiallength of the mold. One or more removable dividers can be positioned inthe mold, either individually or as an assembly. The divider can becomposed of any material that will sufficiently retain its shape andposition during the soot fill and divider removal steps, such as, butnot limited to, card stock, foil, Teflon, or high density polyethylene.The divider can form a boundary at one interface, or can be constructedas an assembly to provide a plurality of concentric layers.

Next, step 108 of consolidating the optical fiber preform is performed.Step 108 may include dehydrating, or drying, the optical fiber preform,and heating of the preform. The optical fiber preform may be heated to atemperature greater than about 800° C., 1000° C., 1200° C., or greaterthan about 1600° C. such that the pressed soot is allowed to sintertogether, or consolidate. Isostatic pressure may be applied during theconsolidation step. Next, step 112 of drawing an optical fiber from theconsolidated optical fiber preform is performed. Step 112 may beperformed using a fiber draw furnace and its associated hardware toproduce an optical fiber.

It will be understood that a variety of advantages may be derived fromuse of this disclosure. For example, the efficient molecular mixing ofthe first and second plurality of particles 20A, 20 b may be achievedthrough mixing of the exhaust gas from the primary and secondary burners14, 18 eliminating the necessity for post-production mixing of the firstand second plurality of particles 20A, 20B using a single set up.Further, the efficient molecular mixing disclosed herein providesmaterial particularly well suited for pressing to form an optical fiberpreform as explained above and below. Even further, the bimodaldistribution of the silica particles may provide greater strength (e.g.,increased tensile strength and/or reduced elastic modulus) to theoptical fiber preform during preprocessing steps (e.g., drying,consolidation and/or preheating).

Examples

FIG. 5 depicts a graph of the diameter of particles (e.g., the secondplurality of particles 20B) with respect to temperature for severaldifferent times and temperatures the particles undergo. The curvesdemonstrate the largest and smallest particles generated in a samplebased on time at a temperature. For example, the largest particlesproduced after 1 second is given by the curve 1L and the smallestparticles produced at 0.1 seconds is given by the curve 0.1S. Further,the average surface area of a sample for a given time at a temperatureis provided. For example, the average surface area of a sample ofparticles after 1 second is given by the curve SA1. As can be seen, thedistribution of particle sizes increases with increasing temperature andsharply divides after a threshold temperature is reached where particlesbegin to sinter together to form larger particles. Further, it can beseen that a low particle size and a high surface area per gram of sample(i.e., low particle size) is maintained at higher temperatures when thetime is kept shorter. As can be seen, in order to generate sub 30 nmparticles, the residence of the particles (e.g., the second plurality ofparticles 20B) should be less than 0.1 seconds at 1650° C., or less than1 second at 1450° C.

FIG. 6 depicts a fluid flow simulation of a large particle burner (e.g.,the primary burner 14) and a small particle burner (e.g., the secondaryburner 18). The small particle burner is configured to flow the exhaustgas and small particles (e.g., the second plurality of particles 20B)into the exhaust and large particles (e.g., the first plurality ofparticles 20A) of the large particle burner (e.g., the primary burner14). As can be seen in the simulation, the majority of the smallparticle burner has a temperature below about 1600° C. such that thesmall particle burner is capable of producing sub 10 nm particles.

FIG. 7 depicts the tensile strength of a plurality of green bodies(e.g., unconsolidated optical fiber preforms) formed of small particles.Sample A consists of a plurality of silica particles having a surfacearea of 22 m²/gram. Sample B consists of a bimodal silica having 7% byweight of 7 nm silica particles and the balance consisting of silicaparticles having a surface area of 22 m²/gram with the different sizesof silica particles mixed after formation. Sample C consists of abimodal silica having 7% by weight of 7 nm silica particles and thebalance consisting of silica particles having a surface area of 22m²/gram with the different sizes of silica particles mixed proximate thetime of formation consistent with the above disclosure. As can be seen,the use of bimodal silica that has been mixed at the time of formationincreases the tensile strength of the green body at all of thetemperatures tested compared to the uniform size of the silica particlesof Sample A.

FIG. 8 depicts the strain tolerance of green bodies, or the strengthdivided by the modulus multiplied by 1,000. Samples A-C are consistentin particle size with the samples of FIG. 6. As can be seen, the straintolerance of sample B is the highest over the given temperature range ascompared to samples A and C. The strain tolerance of the samples isimportant as it helps control how the green body formed of the samplewill perform during heating and sintering (e.g., consolidation of theoptical fiber preform). The strain tolerance of the optical green bodyshould be high such that the optical green body has good thermalsurvivability during consolidation. Other properties may also beimportant such as low contamination and high degrees of molecular mixingwithin the bimodal soot. The small particles (e.g., the second pluralityof particles 20B) present may be in the form of agglomerates and tend toact like springs. When pressed the small particles may tend to bend. Thebending of the small particles may give rise to a lower elastic modulusand thus gives the optical green body more resistance to breaking whenstressed during consolidation.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the claims.

What is claimed is:
 1. A method of producing bi-modal particles,comprising the steps: igniting a first precursor gas using a primaryburner thereby producing a first plurality of particles of a first size;fluidly transporting the first plurality of particles down a particletube; igniting a second precursor gas using a secondary burner therebyproducing a second plurality of particles of a second size; flowing thesecond plurality of particles into the first plurality of particles; andcapturing the first and second plurality of particles.
 2. The method ofclaim 1, wherein the first plurality of particles comprise SiO₂ andexhibit a particle D50 diameter of greater than about 25 nm and thesecond plurality of particles comprise SiO₂ and exhibit a particle D50diameter of less than about 25 nm.
 3. The method of claim 2, wherein thesecond plurality of particles exhibit a particle D50 diameter of lessthan about 10 nm.
 4. The method of claim 3, wherein the second pluralityof particles exhibit a particle D50 diameter of less than about 6 nm. 5.The method of claim 1, wherein the second precursor gas has a higherflow rate than the first precursor gas.
 6. The method of claim 1,wherein the second precursor gas comprises diatomic hydrogen and anorganic siloxane.
 7. The method of claim 1, wherein the step of ignitingthe first precursor gas results in the first plurality of particlesbeing formed at a higher temperature than the ignition of the secondprecursor gas forms the second plurality of particles at.
 8. The methodof claim 7, wherein the first plurality of particles is formed at atemperature greater than about 1800° C. and the second plurality ofparticles is formed at a temperature of less than about 1500° C.
 9. Aburner system, comprising: a primary burner positioned within a particletube, the primary burner configured to produce a first plurality ofparticles; a secondary burner positioned within a secondary tube, thesecondary burner configured to produce a second plurality of particles,wherein the particle tube and the secondary tube are fluidly connectedsuch that the first plurality of particles and the second plurality ofparticles are mixed; and a bag house configured to collect the first andsecond plurality of particles.
 10. The burner system of claim 9, whereinthe second plurality of particles exhibit a particle D50 diameter ofless than about 10 nm.
 11. The burner system of claim 10, wherein thefirst plurality of particles exhibit a particle D50 diameter of greaterthan about 25 nm.
 12. The burner system of claim 9, wherein thesecondary tube is positioned downstream of an elevated temperature zoneof the primary burner.
 13. The burner system of claim 9, wherein thesecondary burner comprises a body and a precursor tube, the bodydefining an annulus around the precursor tube.
 14. The burner system ofclaim 13, wherein the secondary burner comprises one or more pilotlights.
 15. A method of forming an optical fiber, comprising the steps:igniting a first gas using a primary burner thereby producing a firstplurality of particles of a first size; igniting a second gas using asecondary burner thereby producing a second plurality of particles of asecond size; flowing the second plurality of particles into the firstplurality of particles; capturing the first and second plurality ofparticles; and pressing the first and second plurality of particles intoan optical fiber preform.
 16. The method of claim 15, further comprisingthe steps: consolidating the optical fiber preform; and drawing anoptical fiber from the optical fiber preform.
 17. The method of claim16, wherein a flow rate of the second gas is controlled such that thesecond plurality of particles spend less than about 10 seconds above atemperature of about 1300° C.
 18. The method of claim 17, wherein thesecond plurality of particles exhibit a surface area of at least about225 m²/g.
 19. The method of claim 18, wherein the second plurality ofparticles exhibit a surface area of at least about 350 m²/g.
 20. Themethod of claim 19, wherein the second gas comprises a combustible gasand an organic siloxane.